Specifications

Communications 3-4
inherent in its structure. The daisy-chain is a series-connected structure and
any break in the communications link will bring down the whole system.
Several rules must be observed to create a working chain:
1. All wiring connections must be secure; any break in the wiring,
power, ground or communications breaks the chain.
2. All modules must be plugged into their own connectors.
3. All modules must be setup for the same baud rate.
4. All modules must be setup for echo.
Software Considerations
If the host device is a computer, it must be able to handle the echoed
command messages on its Receive input along with the responses from the
module. This can be handled by software string functions by observing that
a module response always begins with a ‘ * ‘ or ‘ ? ‘ character and ends with
a carriage return.
A properly addressed D1000 module in a daisy chain will echo all of the
characters in the command including the terminating carriage return. Upon
receiving the carriage return, the module will immediately calculate and
transmit the response to the command. During this time, the module will not
echo any characters that appear on its receive input. However, if a character
is received during this computation period, it will be stored in the module’s
internal receive buffer. This character will be echoed after the response
string is transmitted by the module. This situation will occur if the host
computer appends a linefeed character on the command carriage return. In
this case the linefeed character will be echoed after the response string has
been transmitted.
The daisy chain also affects the command timeout specifications. When a
module in the chain receives a character it is echoed by retransmitting the
character through the module’s internal UART. This method is used to
provide more reliable communications since the UART eliminates any
slewing errors caused by the transmission lines. However, this method
creates a delay in propagating the character through the chain. The delay
is equal to the time necessary to retransmit one character using the baud
rate setup in the module:
Baud Rate Delay Baud Rate Delay
300 33.30ms 9600 1.04ms
600 16.70ms 19200 0.52ms
1200 8.33ms 38400 0.26ms
2400 4.17ms 57600 173.6µs
4800 2.08ms 115200 86.8µs
One delay time is accumulated for each module in the chain. For example,